1. Field of the Invention
This invention relates to methods of joining components.
More particularly the invention relates to methods of joining a component of molybdenum and/or tungsten to a component of silicon.
2. Description of Related Art
Such methods find application in the manufacture of high power silicon semiconductor devices. In such devices the electrical connections to the silicon wafer in which the device is formed are conventionally made by way of relatively massive copper electrodes, one on each side, which also serve to conduct heat away from the wafer. Ideally the wafer would be bonded directly to these copper electrodes. In practice the difference between the thermal expansion coefficients of copper and silicon renders this impracticable. Instead, one main face of the wafer is conventionally brazed to a molybdenum and/or tungsten backing plate, normally in the form of a disc, and a dry sliding contact between one copper electrode and the disc is made. The other copper electrode makes a dry sliding contact with another molybdenum and/or tungsten disc which in turn makes a dry sliding contact with the main face of the wafer remote from the backing plate, the wafer being unable to withstand the stress associated with two brazed joints.
The silicon wafer is typically joined to the backing plate using an aluminium-based braze. One example of such a braze is the well known Al-12% wt Si braze having a melting point of 577.degree. C. Other examples of such a braze are the well known aluminium-copper-silicon brazes which have a melting point of about 525.degree. C.
When using such aluminium-based brazes to join a silicon component to a component of molybdenum and/or tungsten the brazing operation is typically carried out at a temperature in the range 680.degree.-700.degree. C., i.e. well above the melting point of the braze, for the following reason.
The aluminium-based brazes will not uniformly wet a bare molybdenum or tungsten surface, but at temperatures above about 650.degree. C. a continuous layer of molybdenum or tungsten disilicide forms at the interface between the braze and the disc which the braze can wet.
This relatively high brazing temperature increases dissolution of silicon in the braze which produces two phenomena harmful to device performance.
Firstly, the resulting erosion at the silicon wafer surface tends to be irregular with deep asperities developing in local areas. This is clearly potentially detrimental, especially when semiconductor junctions exist close to the surface of the silicon wafer to be brazed.
Secondly, the dissolved silicon saturated with aluminium precipitates on cooling below the brazing temperature. Most of this silicon forms as a regrowth layer on the eroded surface of the silicon wafer, and because it is saturated with aluminium, the regrowth layer will exhibit p-type conductivity. If the silicon wafer has an n-type doping surface, an extraneous p-n junction will therefore be formed in the wafer between the regrowth layer and undissolved silicon which may be harmful to device peformance.